FIELD OF THE INVENTION
This invention relates to a gas turbine vane. In particular, it relates to a gas turbine vane structure which must be cooled, and which is used as a first stage for an industrial gas turbine engine.
In a general industrial gas turbine engine, a selfdriving system has been adopted in which a turbine directly drives a compressor to supply air to a combustion apparatus. A most effective method to increase the output efficiency of the gas turbine is to increase the combustion gas temperature. However, the combustion gas temperature is restricted by thermal stress resistivity, high temperature oxidation resistivity or corrosion resistivity of the turbine vane. More specifically, the temperature is restricted by the materials comprising the stationary and rotary vanes used in the first stage.
Thus, in a conventional gas turbine, vanes are provided with a cooling structure to cool the vane from the inside using a coolant fluid, as shown in FIG. 6. FIG. 6 shows an example of a first stage stationary vane of a gas turbine, and is a longitudinal sectional view taken on a camber line of a vane body. FIG. 7 is a transverse sectional view taken on a line A--A of FIG. 6. This vane is composed of a vane airfoil i, an upper end wall 2 and a lower end wall 3. A cavity 4 extended along the longitudinal direction of the vane airfoil 1 is formed within the vane airfoil 1. A guide cylinder 5 to guide the coolant fluid is supported on the upper end wall 2 and is disposed into the cavity 4.
The coolant fluid enters the guide cylinder 5 from an impingement plate 6 and cools the upper end wall 2. A part of the coolant fluid flows out from upper film cooling holes 7 and film-cools the surface of the upper end wall 2. The remaining coolant fluid is led to the guide cylinder 5 and flows out from impingement holes 8 drilled along the whole surface of the longitudinal direction. This fluid impingement-cools an inner surface 9 of a leading edge of the vane. As shown in FIGS. 6 and 8, protrusions 10 disposed parallel to each other in a chord direction are provided on an inner surface of the vane airfoil 1. Protrusions 10 have rectangular sections and are arranged parallel and at the same intervals to each other.
The lengths of protrusions 10 are substantially the same as the width of the guide cylinder 5. Protrusions 10 disposed on the inner surface of the vane airfoil 1 and the outer surface of the guide cylinder 5 are adhered closely. Cooling ducts 11 are defined as the spaces surrounded by the inner wall of the vane airfoil held between adjacent protrusions 10, side walls 15 of the protrusions 10 and the outer surface 16 of the guide cylinder 5. The coolant fluid impinging on the inner surface of the leading edge of the vane airfoil I flows to the trailing edge of the vane. As a result, the coolant fluid convection-cools the vane airfoil 1 from its inner surface, and flows out of the vane through gaps between pin fins 12 formed on the trailing edge to accelerate the convection effect.
In the lower end wall 3, similarly, the coolant fluid entering from an lower impingement plate 13 impingement-cools the lower end wall 3. Thereafter, the coolant fluid flows out from lower film cooling holes 14 and film-cools the surface of the lower end wall 3.
However, there are several problems with the above-mentioned conventional vane. Namely, the coolant fluid temperature rises considerably when the coolant fluid reaches the trailing edge through cooling ducts 11. As a result, the cooling effect is decreased on the trailing edge. Besides, the temperature distribution of the main flow of combustion gas is shown in FIG. 9. Furthermore, the thermal stress also increases because the temperature distribution of the vane airfoil grows larger. Therefore, the temperature of the vane surface also becomes highest near the center area in the longitudinal direction of the vane, and the temperature distribution of the vane surface is spread widely.
It is necessary to maintain the average temperature of the vane airfoil 1 and the maximum temperature at the limited part at a permissible value. Therefore, if a cooling design is performed to keep the temperature of the center area of the vane within the permissible value, the upper and lower sides of the vane become supercooled. As a result, effective cooling is not performed, because the imbalance of the thermal distribution causes thermal stress. Thus, the above-mentioned conventional vane has a problem in not being able to be cooled with a small temperature difference on the vane surface using effectively the coolant fluid.